There are many characteristics of absorbent paper products, such as towel and tissue product, that must be considered in producing a final product having desirable attributes that make it suitable and preferred for the product's intended purpose. Absorbency, strength (both wet and dry), and softness are among the most important and desirable characteristics of disposable paper towels. Absorbency is the ability of a product to absorb and retain liquid. Overall absorbency as perceived by the consumer is generally considered to be a combination of the total quantity of liquid a given mass of paper will absorb at saturation, as well as the rate at which the mass absorbs the liquid. Strength is the property of a paper product that causes the product to be held together while in use. Finally, softness is the product property accounting for the pleasing tactile sensations imparted to the human anatomy while the product is in use. In general, the major easily measured determinants of perceived softness include stiffness and bulk (density), with lower stiffness and higher bulk (lower density) generally improving perceived softness and/or tactile perception of the products.
These three product attributes often run counter to one another. For example, as strength is increased, softness and absorbency generally decrease. Consumer product companies and vendors are constantly searching for methods/materials to either increase softness and absorbency while maintaining or increasing strength or to increase strength while maintaining or increasing absorbency and softness.
To improve softness, standard paper making processes often add chemicals, e.g., softeners and debonders, to a fiber furnish or web to improve or change the properties of the web. Traditionally, softeners and debonders are used in the papermaking process to enhance softness or to adjust strength. Typically, these chemicals are added to the wet end of the paper making process, i.e., in the paper making slurry. When used in this manner, these chemicals can react with fines, pitch, sand and other materials associated with pulp fibers to form deposits. Deposits negatively impact productivity because they blind fabrics and plug felts. Significant expenses must be incurred to remove the deposits. In some instances, the presence of these chemicals requires adjustment of the system pH. Because of the fatty acid groups, hydrophobicity is imparted on the paper product and this renders it non-absorbent. In some cases, additional expense must be incurred when hydrophilic surfactants are used to restore or impart absorbency. In some cases, it will be desirable to effect significant reductions in ultimate tensile strength. These are most easily obtained when debonders are added at the wet end.
These chemicals have occasionally been sprayed onto the wet web prior to drying. These processes can suffer from the disadvantages of contamination and materials loss while overspray and mist created during the spraying process can be irritating to the paper machine attendants. Unretained chemicals applied in this manner are usually recirculated back to the wet end where they may also react with fines, pitch, sand, and other materials associated with the pulp fibers to form deposits and may require pH adjustment. Deposits negatively impact performance and are expensive to remove and clean up. Adding debonders by spraying is not usually as effective in reducing ultimate tensile strength as is wet end addition.
The use of wet strength agents to enhance the wet strength of a paper web is widely known. For example, Westfelt described a number wet strength agent materials and discussed their chemistry in Cellulose Chemistry and Technology, Volume 13, at pages 813-825 (1979). Freimark et al. in U.S. Pat. No. 3,755,220 issued Aug. 28, 1973 mention that certain chemical additives known as debonding agents interfere with the natural fiber-to-fiber bonding that occurs during sheet formation in paper making processes. This reduction in bonding leads to a softer, or less harsh, sheet of paper. Freimark et al. go on to teach the use of wet strength agents in conjunction with the use of debonding agents to off-set the undesirable effects of the debonding agents on the paper strength. These debonding agents reduce both dry tensile strength and wet tensile strength.
To impart greater softness to the paper product, styrene-butadiene latexes can be used as the binder system. However, these styrene-butadiene latexes are usually either nonionic in character or else are partially anionic due to inclusion of anionic comonomers or surfactants. The nonionic styrene-butadiene latexes cannot be used as “wet-end additives” in a conventional papermaking process. Instead, these nonionic latexes have to be impregnated or pattern-printed on the subsequently laid paper furnish, such as by the process described in European Patent application 33,988 to Graves et al, published Aug. 19, 1981.
An anionic styrene-butadiene latex can be used in a conventional wet-end additive papermaking process by adding a cationic polyelectrolyte. See, for example, U.S. Pat. No. 4,121,966 to Amano et al, issued Oct. 24, 1978; and U.S. Pat. No. 2,745,744 to Weidner et al, issued May 15, 1956. The cationic polyelectrolyte used is typically a water-soluble cationic wet-strength agent. Basically, the cationic polyelectrolyte, when added, destabilizes the dispersed anionic latex particles which then flocculate and deposit on the paper fibers. Accordingly, the cationic polyelectrolyte and anionic styrene-butadiene latex cannot be combined together until the point at which they are used as the binder system in papermaking.
Polyethylene glycols (PEG) and PEG fatty esters and ethers are also used as wetting agents and plasticizers for paper products. Unfortunately these PEG derivatives are not substantive to the cellulose fibers. And the process involved to incorporate the PEG into the paper can be problematic and expensive.
U.S. Pat. No. 5,240,562 to Phan et al. issued on Aug. 31, 1993 and discloses the use of preformed vesicles as a means of carrying these non-substantive PEGs into the final paper substrate. However, these vesicles require preparing, maintaining and delivering complex formulations at high temperatures to the wet end of the paper machine.
U.S. Pat. No. 3,434,984 to James Hyland issued on Mar. 25, 1969 discloses thermosetting cationic resins which impart wet strength and water holdout properties to cellulosic materials. These resins are prepared by reacting epichlorohydrin and a polyamide. However, these materials can only be incorporated into the web at high temperatures of up to 200° C.
There is a need in the paper industry to provide a paper product and a method for making the product where incorporation of wet strength agents can be done efficiently and at room temperature while enhancing the softness and absorbency of the paper product.
The present invention provides an absorbent paper product that overcomes the disadvantages associated with the prior art technologies.
The present invention also overcomes the disadvantages associated with the prior art technologies by providing a method for making soft and absorbent wet strengthened paper products.